It is sometimes advantageous to couple optical radiation into or out of a waveguide at a point other than at an end. For instance, which jointing optical fibres it is known to align the fibre ends to be jointed by feeding light from one fibre end into the other and maneuvering them relative to one another to maximise coupling between the fibre ends. To do so, a bend is produced in the fibres to each side of the jointing region and the light is injected at the bend in one fibre and monitored by picking it off at the bend in the other fibre.
Although this technique for injecting and picking off light has the advantage that little preparation of the fibre has to be done, that is, it can be carried out for instance through a protective polymer coating carried by a fibre, there are circumstances in which it is unsuitable. The technique injects a relatively high power level of light and picks off as high a proportion of the light injected as possible. There are applications in which it is still advantageous to pick off radiation at a point along a fibre other than at an end, but in which it is preferable that only low power levels are picked off. For instance, in an optical fibre communications system, one may wish to detect a malfunctioning fibre amongst working fibres. If one were to test for the malfunctioning fibre by picking off high power levels from random fibres, the transmission of one or more working fibres is likely to be interfered with.
Optical couplers are known which can be used to couple an adjustable power level of light into or from an optical fibre but they rely on careful preparation of the fibre. For instance, in one type, the core glass of the fibre has to be brought close to the fibre surface by mounting the fibre in a curved position and polishing away the cladding glass on the outside of the curve until fibre core glass is almost exposed. Light is then coupled into and out of the exposed core of the fibre by controlling the relative position of a second fibre which has been polished in the same manner.
Optical couplers of this type suffer from the disadvantages that they are time-consuming and expensive to produce. A monomode fibre typically has an outer diameter of 125 .mu.m and a core diameter of only 9 .mu.m. Hence it is difficult to control the polishing steps with sufficient accuracy. The couplers are also limited in that they can only be used at predetermined fixed positions along a prepared fibre. Further, the fact that the arrangement tends to be vulnerable and unsuitable for use in the field.
It is an object of the present invention to provide an optical coupling device which can conveniently be used to test optical fibres in use in an optical communications system.
According to the present invention there is provided an optical coupling device, for coupling low power levels of optical radiation out of an optical waveguide having a refractive index of n.sub.1 at or near its surface, comprising a pick-up element having a refractive index n.sub.3 and providing a curved optical waveguiding path therein, and clamping means for clamping the optical waveguide into a curved position which at least substantially conforms to the inner side of the curved path, the refractive indices of the pick-up element and the surface of the optical waveguide, n.sub.3 and n.sub.1, and the radius of curvature, R, in mm of the inner surface of the curved path portion of the pick-up element being chosen such that n.sub.3 &gt;n.sub.1 (1-0.125/R): such that optical radiation is leaked from the optical waveguide in use, and coupled into the curved path.
It has been found that a relatively simple coupling device can be designed according to embodiments of the present invention which will pick up sufficient radiation from for instance an optical fibre to check whether the fibre is functioning, without significantly disturbing a communications system dependent on the functioning of the fibre.
A waveguiding path in this context is intended to mean a path along which optical radiation will be guided by means of the distribution of refractive index in the materials of the waveguiding path. In general, a waveguiding path will comprise in cross section a core region of one refractive index, surrounded by a cladding region of a lower refractive index or range of refractive index.
In order that optical radiation coupled into the curved path does not return to the optical waveguide, the refractive index of the material of the path should preferably be greater than that of the optical waveguide.
Conveniently the pick-up element may comprise a solid curved rod of a dielectric material such as silica. The clamping means may act to clamp the optical waveguide against the inner side of the curved path; for instance where a rod is used, against the inner side of the curved rod.
The clamping means may comprise a block of material having a convex surface which corresponds in shape to the inner side of the curved, waveguiding path, a groove being provided in the convex surface to locate the optical waveguide, and means to hold the block and pick-up element together so as to grip an optical waveguide positioned in the groove.
The radiation from a curved waveguide does not leak away uniformly with distance along the waveguide, but in a series of discrete, well-defined, tangential beams. These beams form a divergent pattern, angular separation of the beams being a function of bend radius of the waveguide. Both the bend radius and the length of the optical waveguide which is curved are factors which affect the power level of optical radiation coupled out of the optical waveguide. For optical communications systems, using 1300 nm radiation transmitted by monomode fibres, a suitable radius of curvature of a fibre to couple out sufficient power for testing purposes without significantly degrading transmission has been found to be of the order of 15 mm, the total length of curved fibre subtending an angle of about 120.degree.. Using such an arrangement, the loss introduced to the system may be of the order of only 3 dB.
We have also found that efficient coupling is facilitated by extending the pick-up element so that it and the optical fibre are aligned not just over the aforesaid curved portion, but also over a straight path thereafter. Although the length of the straight portion is not finely critical, we have found that optimum performance is obtained when it is about twice the diameter of the bend. Shorter lengths can be used of course but satisfactory coupling out of the cladding modes is increasingly difficult to achieve as this length is reduced. Greater lengths can be used where they are not inconvenient and where attenuation in the pick-up element is not a problem, but in general little useful power remains in the cladding much beyond the double diameter point.
It has been found that devices according to embodiments of the present invention can couple power levels of optical radiation out of an optical fibre which has a primary protective plastic coating intact as low as about 1 nW. To avoid significant interference with a monomode fibre communications system, typically the power level of the optical radiation coupled out of a fibre should be less than or equal to 50% of the average power level of optical radiation being transmitted by the fibre.
A coupling device may conveniently be applied to an optical fibre at a point in a communications system where the primary coating of the fibre is exposed for routing purposes. For instance, this is generally the case in joint housing, and at distribution points.